Various fluid flow measuring devices and methods are known for use with fluid flow systems to measure a differential pressure across a piece of equipment. Based on the known characteristics of the equipment, the observed differential pressure can be converted into a fluid flow rate.
One common device used to measure fluid pressure is a pitot tube. The pitot tube may be positioned to measure fluid pressure at a specific point within a fluid line. The pitot tube typically includes an opening that may be placed in the stream of the fluid flow. To obtain an accurate reading, the pitot tube must be positioned so that the opening is located in the exact center of the stream and oriented parallel to the fluid flow path. A gauge may be operably coupled to the pitot tube to provide a pressure reading.
In a pitot tube, static pressure, which is atmospheric pressure in an open system, is compared to total pressure. Using Bernoulli's Equation, the pressure differential is then translated to a flow velocity. A gauge device may be graduated such that the readout is directly displayed as a fluid velocity.
Pitot tubes are commonly used where there is a desire to determine the flow rates of fluids for equipment testing. Fire pumps, in addition to other pumping equipment, often must be tested to ensure the equipment meets certain performance specifications, and therefore it is important to provide accurate devices and methods for determining fluid flow rates.
Many fire hydrant flow tests are conducted by taking a pitot reading directly from the nozzle on the fire hydrant. Due to inexact orifice diameter, excessive turbulence (which may cause fluctuations in the observed pressure level of +/−10 psi), and incorrect pitot tube positioning, these options give the least dependable readings.
Some devices integrate the pitot tube into the fluid line. A general problem with pitot tubes is that they are difficult to position to obtain accurate pressure readings. As noted above, the pitot tube must be positioned in the center of fluid flow and oriented parallel to the flow. As the pitot tube placement deviates from these requirements, the less accurate the pressure readings will be. Additionally, the pitot tube structure extends partially into the fluid line, and therefore is susceptible to damage by solids entrained in the fluid.
Conventional pitot tubes and other flow testing apparatus are also susceptible to inaccuracies due to turbulent fluid flow. Turbulence may be generated by valves, elbows, or other components that disrupt the flow of fluid through the pipe. Consequently, conventional devices typically require minimum lengths of straight pipe upstream and downstream of the flow testing apparatus to reduce turbulence and therefore improve accuracy of the data obtained by the flow testing apparatus.
U.S. Pat. No. 4,555,952 to Jenkins, issued on Dec. 3, 1985, discloses a differential pressure sensor. The pressure sensor responds to pressure differential across an orifice of known size. Here, the fluid pressure is measured on one side of an orifice plate. The fluid flows through the orifice. The pressure is also measured on the other side of the orifice plate. The two pressures are compared across a diaphragm. The pressure differential is transmitted to an electronic transducer located a distance safe enough away to protect the transducer from the temperature of the fluid. Thus, to measure pressure differential, the '952 patent requires an additional energy-consuming and turbulent-producing orifice plate which can affect the accuracy of the measurement.
U.S. Pat. No. 4,343,193 to Holden, issued on Aug. 10, 1982, discloses a flow measuring apparatus. The Holdren device measures pressure differential across a removable orifice plate. When a reading is not required, the orifice plate can be removed from the system piping. Again, the orifice plate of Holdren causes turbulence and consumes fluid energy, thereby decreasing the accuracy of the pressure readings.
U.S. Pat. No. 2,564,272 to Morton, issued on Aug. 14, 1956, discloses a flow meter attachment for hose nozzles. The nozzle is accompanied with a gauge tapping for a pressure gauge. The flow meter attachment includes two tubular sections. The inner tubular section has a plurality of holes extending through the wall such that there is fluid communication between the inside and outside of the inner tubular section. The gauge measures the pressure differential between the two tubular sections and the atmosphere. However, the Morton device does not allow for full measurement of the velocity component of Bernoulli's Equation since there is diminished flow in between the two tubular sections, thereby resulting in a less accurate reading. In addition, the '272 patent requires that the characteristics of the nozzle be known, such that the pressure gauge can be calibrated directly in fluid flow.
More recently, the present applicant developed a pitotless nozzle disclosed in U.S. Pat. No. 6,874,375 to Grenning, which issued on Apr. 5, 2005. The pitotless nozzle includes a constant pressure nozzle that promotes laminar flow through the nozzle. Laminar flow has a relatively constant pressure profile, and therefore an accurate pressure reading may be obtained from a periphery of the flow, without requiring components that obstruct the fluid flow. The pitotless nozzle, however, is limited to use in so-called “open” systems, where the pitotless nozzle is located immediately prior to the point at which the fluid is discharged from the fluid line to atmosphere. In applications where the fluid line to be tested does not have a nearby drain, a hose or other conduit must be attached to the fluid line and routed to a point where fluid may be discharged. The pitotless nozzle is attached to the end of the hose, and therefore the fluid line valve control may be remote from the pitotless nozzle. Consequently, to run a flow test, the user must open the fluid line control valve, move to the discharge point to observe the test data, and return to the fluid line control valve to close it once the test is complete. Requiring a user to travel between the fluid line control valve and the discharge point at the beginning and end of the test results in wasted time and fluid. Alternatively, two individuals having radio or other communication means must be used to run the test, necessitating otherwise unnecessary communication equipment and expending additional manpower.